Unusual weather pattern continues across U.S.

The storm system that brought record late-season snows to parts of the Plains and Midwest continues to live on, and is currently drifting eastward across the southern U.S.  Temperatures are too warm for snow at this point, but prolonged heavy rain will affect parts of Tennessee, Alabama, and Georgia early this week.  The big story with this system continues to be nearly-unprecedented cold temperatures for this time of year.  Numerous locations across Texas, Arkansas, Louisiana, and Mississippi set records this morning not only for daily lowest temperature, but for lowest temperature recorded in the month of May.  Houston and College Station both fell to 42º this morning, easily breaking their old records for coldest May temperature.

The culprit behind the record cold is a cut-off low-pressure system drifting across the U.S.  This system broke away from the main steering flow, or jet stream, mid last week, and now without a steering mechanism, this system can only crawl along.  The entire upper-level weather pattern over North America is essentially "blocked", with the main jet stream well to the north in Canada, and several strong lows and highs staggered across the United States (see image on right).  The big doughnut over the South is the cut-off low causing record cold and heavy rain.  Another cut-off low is located over northern California, which will bring cooler temps and scattered thunderstorms early this week - welcome relief to the recent spate of hot temperatures and wildfires (assuming the thunderstorms themselves do not start additional wildfires).

Meanwhile, two strong high pressure systems are located over the Pacific Northwest and the Great Lakes.  These regions are experiencing sunny skies with above-normal temperatures.  Locations such as Seattle, Detroit, and Toronto will be warmer compared to many cities further south such as San Francisco, Atlanta, and Memphis early this week.  The position of these lows and highs relative to one another is causing the atmospheric "blocking".  There are two main types of blocking patterns - a Rex Block, which is a high pressure system over (poleward of) a low pressure system, and an Omega Block, which consists of a high pressure ridge flanked by a low pressure system on either side and looks somewhat similar to the Greek letter Omega (Ω).  A schematic of these blocking patterns is shown on the right.  Both patterns are stable patterns, meaning they can remain in place for a long

time.  For example, in the Rex Block pattern, the circulation around the high to the north opposes feeds into the circulation of the low to the south, and vice versa.  Thus, as long as the high and low are of relatively equal strength, there is little movement to the west or east.  Right now, we have forms of both blocking patterns over the U.S., with a large pseudo-Omega block (dashed blue circle) consisting of a high pressure ridge over the Plains flanked by the cut-off lows over the Southeast and California, and then the two cut-off lows are themselves part of Rex Blocks (dashed red circles) with the high pressure systems over the Pacific NW and the Great Lakes.  This pattern will be very slow to break down and likely won't happen until late in the week when forecast models indicate a strong jet stream over the north Pacific will break into the Rex Block on the West Coast.